I love 2-blade heads, and have shot them exclusively for the past few years, but I still have to mostly agree with Charlie, Terry, Curt, Ryan and the like.
I've killed a bunch of critters with 2-blade heads (whietails, elk, a big S. Texas boar hog, a bull moose and a pile of small game). Penetration's never been an issue, including blowing through my moose after splitting a rib vertically. Out of all those animals, I've only had two bloodtrail problems.
One was with my hog, and quite honestly, had I remembered Curtis' advise about not shooting hogs like deer, I wouldn't have hit it through the liver in the first place. Mea culpla. Charlie, Curtis and I were able to find it the next day after a lot of careful tracking.
The other was a small whitetail buck that decided to take a step and turn toward me just as I dropped the string. I ended up hitting him in through the liver and out through the intestines...which clogged the exit wound. The deer ended up running into a few dozen acres of waist-high prairie grass. Some friends and I found him the next morning after a little grid searching and a lot of luck.
In hindsight, I've also had a few deer over the years that left very sparce trails. One that comes to mind was a double-lung pass through with a small 4-blade head. Another was a slightly quartering away doe I took through both lungs with a small 3-blade model. The first deer I heard expire, so trailing was a moot point. The second involved a lot of hands-and-knees crawling and looking for pinhead spots of blood.
That notwithstanding, all one has to do is look at Terry's diagram to see that there's a heck of a lot more room to go wrong on a deer away from bone than through it. And as much as I'm a big fan of 2-blade heads, if I screw up and hit something behind the diaphragm, I'd rather have a broadhead that resembles a meat cleaver than a pocket knife.